Reexamining sexual and species recognition in butterflies: the Partner-Hazard Space framework

Generally, intra- and inter-sexual interactions have been considered qualitatively different processes. However, same-sex sexual behavior is widespread in the animal kingdom, challenging this assumption. Recently, a hypothesis was proposed that regards territorial contests of butterflies, where two males chase each other without apparent physical attack, as an initial phase of mating behavior. 

Building on this idea, an Osaka Metropolitan University-led research team reviewed research on sexual and species recognition in butterflies to assess the extent to which they recognize the same sex and species. The team found no evidence that butterflies recognize the same sex, and consequently, no evidence that they recognize their own species (the set of the same sex and the opposite sex). 

Phenomena previously interpreted as sexual or species recognition were indistinguishable from those interpreted as mate choice. In addition, although butterflies tend to escape from looming objects, there is no evidence that they recognize predators at the species level. Thus, known butterfly interactions can be explained by assuming that they possess a sexual-partner template (that elicits mating behavior) and a hazard template (that elicits escape responses). 

Within this framework, which the team termed the Partner-Hazard Space (PHS), other animals can be located in a space according to their similarity to sexual-partner and hazard templates. Under PHS, intra- and most interspecific interactions of butterflies can be understood as various phases of sexual behavior or escape responses to hazards. PHS provides a parsimonious hypothesis for interpreting butterfly interactions without assuming recognition of sexual rivals or species. 

Furthermore, PHS also provides a simple explanation for the evolution of conspicuous sexual and species-specific traits in butterflies. Since animals must identify mates from a broader pool of individuals, including individuals of the same species (conspecifics) of both sexes and individuals of different species (heterospecifics), sexual ornaments convey vital information: “I am a sexual partner.” Under such circumstances, preference for these traits confers the direct benefit of securing a mate of the conspecific opposite sex.

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